Current approaches to measuring the proximity of two devices, such as a mobile phone and a radio beacon, use received signal strength indications (RSSI). The power of a RF signal received at a particular mobile device from a RF beacon is measured. The higher the received signal, the higher the RSSI value and the closer the device is considered to be from the beacon. Mobile devices are polled by the beacon using a unique identifier, which can be used by a mobile device to determine the beacon from which the signal was transmitted and, using the signal strength, the proximity of that beacon relative to the mobile device.
Whilst RSSI provides an approximation of distance of a mobile device from a RF beacon, these measurements are not accurate enough for many applications. For example, in “touch” interactions, where a mobile device is brought into very close proximity of a “touch” terminal, a proximity measurement accurate to less than 1 cm is desirable. Such accuracy cannot be obtained by RSSI measurements, which are highly dependent on noise due to multipath propagation and diffraction interference. Additionally, RSSI sensitivity depends on the sensor chip used and its mounting location in a particular mobile device. Thus, an RSSI measured at a set distance from a sensor will vary from device to device. A further disadvantage of using RSSI in “touch” applications is that the sampling rate of RSSI chips is typically in the region of 1-2 Hz. Since a touch action is usually completed within a two second duration, anomalous readings are hard to guard against without taking more readings for longer, which itself sacrifices user experience.
There is therefore a need for improved techniques for measuring the relative distance between two devices in close proximity, particularly for applications in “touch” technology.
In U.S. Pat. No. 8,683,707, a magnetically modulated location system is described which uses a multiple-dipole magnetic transmitter transmitting on distinct frequencies. The relative angle of the receiver in a single plane can then be ascertained with a two-dipole transmitter.
In PCT Patent Application WO 2014/128703 A1 (Wise-Sec Ltd), an RF signal and a magnetic field are used in order to determine the distance between a mobile device and a target device. Wise-Sec Ltd defines that the detection of an RF signal meeting a certain criterion triggers the device to search for a magnetic field.
Similarly, in German Patent Application DE 10 2012 007 286 A1 (Dahmouni) discloses a device in which detection of an RF signal triggers a device to search for a magnetic field in order to validate the occurrence of an event.